Exhaust gases from internal combustion engines of vehicles and the like contain hydrocarbons, carbon monoxide, and nitrogen oxide, which are harmful to human body and environment. As a catalyst for purifying vehicle exhaust gas, so-called a three way catalyst is used, which oxidizes carbon monoxide and hydrocarbons into carbon dioxide and water, and reduces nitrogen oxide into nitrogen and water. A three way catalyst is composed, for example, of a precious metal such as Pt, Pd, or Rh as a main catalyst, and an oxide or a composite oxide containing cerium oxide as a co-catalyst, both carried on a catalyst support, e.g., of alumina or cordierite. The co-catalyst absorbs oxygen due to change of valency of Ce contained therein from three to four in an oxidizing atmosphere, and desorbs oxygen due to change of the cerium valency from four to three in a reducing atmosphere, which is so-called oxygen absorbing and desorbing capability. This oxygen absorbing and desorbing capability mitigates abrupt change in an exhaust gas atmosphere caused by acceleration and deceleration of an engine, so as to allow the main catalyst to purify exhaust gas at high efficiency. The co-catalyst, while generally exhibiting high oxygen absorbing and desorbing capability at high temperatures, is required to exhibit sufficient oxygen absorbing and desorbing capability even at low engine temperatures, such as at the engine start, in particular, at lower temperatures of 400° C. or lower. Further, the co-catalyst, which carries a main catalyst precious metal thereon, proceeds to sinter when exposed to a high temperature exhaust gas, and as its specific surface area decreases, the main catalyst aggregates, failing to exhibit sufficient catalytic capacity. Thus, the co-catalyst is required to have heat resistance which enables maintenance of a large specific surface area even at high temperatures.
Patent Publication 1 discloses, as a composite oxide usable as a co-catalyst, a zirconium-cerium composite oxide which contains zirconium and cerium at a weight ratio of 51 to 95:49 to 5 in terms of oxides, has a specific surface area of not smaller than 50 m2/g after calcination at 500 to 1000° C., and is capable of maintaining a specific surface area of not smaller than 20 m2/g even after heating at 1100° C. for 6 hours.
Patent Publication 2 discloses a composite oxide which contains Ce and/or Pr and Zr at a particular ratio, is free of tetragonal crystal phases originated from zirconium oxide, and of which electron diffraction pattern appears as dotted diffraction spots. This composite oxide is described to have excellent oxygen absorbing and de sorbing capability at lower temperatures.
Non-patent Publication 1 teaches that enhancing the homogeneity of the Ce and Zr atoms in the CeO2—ZrO2 solid solution increases OSC (oxygen storage/release capacity) performance.    Patent Publication 1: JP-10-194742-A    Patent Publication 2: WO 2009/101984    Non-patent Publication 1: R&D Review of Toyota CRDL, Vol. 37, No. 4, p20-27